And if you talk of cell impedance or inner resistance (IR or Ri) you would normally have to specify if you refer to AC IR or DC IR - that can be (and normally is) a big difference! Actually the IR depends on several parameters: the frequency of measurement and the time of the load (AC or DC method, even the AC frequency is important), the temperature of the cell and the SoC (State of Charge) of the cell. SoC seems to matter the least, whereas temperature is important and most important the frequency. For most datasheets and IR measurements you will find here, they refer to the AC-method, normally 1 kHz - as many IR-testers use 1 kHz AC to measure the IR.
The DC IR is normally significantly higher, typical 2...3 times, but is also more difficult to measure, as the cell gets drained more or less and you get a voltage drop until the reading stabilizes. This can take several seconds - or event won't stabilize at all on a weak cell or to low of a load resistor attached - so that you can't get exact and reproducible readings. Thats why most people use the AC method and typically with 1 kHz.
For the sake of meaningful and comparable data is would suggest to generally use the AC 1 KHz method at 50% SoC (3.7...3.8 V) and at room temperature of 20 deg. C. If the cell is much warmer, the IR can be lower and at very low SoC the IR can be higher.